Ultra-thick wetting layer induced phase separations in P3HT/fullerene solar cells: The femtosecond time-resolved photo-luminescence and the transient absorption

Abstract

Control over morphology in polymer solar cell of conjugated polymer:fullerene active layer has gained experimental interest as the phase separation will change vertical and lateral composition and enhance the optical and electrical properties. While the vertical composition, lateral morphology with electrical or optical charateristics have been observed to support the speculation that surface energy is the driving force for phase separation, the interplay of phase separation and substrate surface energy is elusive. Although researches have suggested the surface-directed spinodal decomposition can be found in polymer:fullerene blends and leads to ideal phase morphology in bulk composition, it is still miscible for the comprehensive analysis for bulk phase morphology, crystallinity acceleration and electronic properties in the device, as well as the dynamic and interplay of phase separation for top surface, bulk domain and wetting area near the substrate. Based on the previous studies, we hypothesize that the formation of wetting layer can be facilitated by controlling the surface energy of substrate, so we design the standard structure device composed of P3HT:PCBM active layer with fluorinated compounds (PFDS) spin-coated on PEDOT: PSS. Experiments designed are to investigate the presence of wetting layer and to observe the vertical composition and lateral phase separation via depth profiling and top surface images. Finally, the time resolved photoluminescence and transient absorption were used to investigate the charge separation and exciton generation and so on.